ABSTRACT Congenital heart defects (CHDs) are the most commonly encountered birth defect, affecting as many as 8 in 1,000 live births. In many cases, life-saving surgical intervention is required. Despite the prevalence, in many cases the underlying molecular etiology of CHDs is not known. The central theme of this Program Project Grant application is to elucidate mechanisms that regulate growth and morphogenesis of the ventricle during development. Our objective is to gain understanding of how early events impacting mesoderm-lineage specification and differentiation, how mid-gestational events impacting septation and papillary muscle formation, and how late-gestational events impacting cardiomyocyte polarity and sarcomere maturation each contribute to normal ventricular morphogenesis. Three highly interactive and complementary Projects are proposed to attain these objectives. Project 1 will study the molecular mechanisms contributing to the genesis of CHDs in an animal model of X-linked heterotaxy. These studies build on work from Dr. Stephanie Ware's laboratory and will test the overall hypothesis that morphogenic defects in early embryonic structures can impact the migration and differentiation of primitive progenitor cells which subsequently give rise to the myocardium. Such a mechanism would explain why the spectrum of CHDs encountered in heterotaxia patients is more severe than what would be anticipated to result from altered sidedness. Project 2 will study the molecular mechanisms regulating ventricular septation and papillary muscle formation. These studies build on work from Dr. Anthony Firulli's laboratory and will test the overall hypothesis that expression cardiomyogenic transcription factors imparts morphogenic cues directing normal cardiomyocyte patterning in the developing left ventricle, and that mid-gestational alteration of this patterning gives rise to CHD. Such a mechanism would establish cell and molecular pathways which regulate normal and abnormal development of the ventricular septation and papillary muscle. Project 3 will study the molecular mechanisms regulating compaction of the left ventricle during cardiac development. These studies build on work from Dr. Weinian Shou's laboratory and will test the overall hypothesis that altered cardiomyocyte cell polarity gives rise to ventricular noncompaction, and furthermore, will dissect the molecular regulatory cascades which are required to establish normal cardiomyocyte polarity. Such a mechanism would establish a common underlying molecular etiology which gives rise to left ventricular noncompaction. The proposed work will be facilitated by the participation of three cores (Administration, Cardiac Imaging and Mouse Resources). Ultimately, the studies proposed in this Program Project Grant application will illustrate how events occurring prior to overt heart formation, during early cardiac development, and during late maturation of the ventricular wall are sequentially integrated for normal cardiac morphogenesis. Importantly, defining the molecular regulation of these events will provide insight into potential interventions aiming to mitigate the deleterious impact of CHD.